3-(3'-fluorenyl-9'-oxo)-L-alanine: a novel photoreactive conformationally constrained amino acid.

Photoaffinity scanning of the ligand-G-protein-coupled receptor bimolecular interface is a direct approach to mapping the interactions of ligands and receptors. Such studies are an important first step toward generating an experimentally based model of the ligand-receptor complex. The synthesis and spectroscopic characterization of Boc-3-(3'-fluorenyl-9'-oxo)-L-alanine and 9-fluorenone-3-carboxylic acid are described. Incorporation of these two photophores into the parathyroid hormone (PTH) molecule yields potent agonists. These photoreactive analogs cross-link specifically with the recombinant human PTH1 receptor stably expressed in human embryonic kidney cells. The availability of the 9-fluorenone (a conformationally constrained derivative of benzophenone, the abundantly used photophore) for photoaffinity scanning provides an important tool to probe the effect of conformational flexibility of the photophore on the selection of the cross-linking site in the macromolecular acceptor.